The present invention relates to cables, in particular for power transmission, for telecommunications or for data transmission, or also combined power/telecommunications cables, wherein the cables comprise materials exhibiting superior mechanical, electrical, and flame-retardant properties.
There is currently a great need for highly environmentally friendly products, consisting of materials which are not harmful to the environment either during their production or when in use, and which are readily recyclable at the end of their working life. However, the option of using ecological materials is, in all cases, subject to the need to keep costs within acceptable limits, while still guaranteeing performance levels which are at least equivalent to those of conventional materials and which are, in any case, satisfactory under the most common conditions of use.
In the cables sector, in particular power transmission cables, the various coatings surrounding the conductor commonly consist of a crosslinked polymer material, in particular polyethylene or ethylene copolymers suitably crosslinked during extrusion, so as to give satisfactory mechanical performances even under heating in continuous use and under conditions of current overload, while at the same time maintaining a high level of flexibility. These materials are crosslinked and therefore cannot be recycled since they are devoid of thermoplastic. properties, hence they can only be disposed of at the end of their working life by means of incineration. Moreover, in certain cases the outer protective sheath consists of polyvinyl chloride (PVC) which is difficult to separate by conventional methods (for example in water by density difference) from the crosslinked polyolefins containing inorganic fillers (for example from ethylene/propylene rubbers containing inorganic fillers), and, on the other hand, PVC cannot be incinerated together with crosslinked polyolefins since this produces highly toxic chlorinated products by combustion.
Patent application WO 96/23311 describes a low-voltage high-current cable wherein the insulating coating, the inner sheath and the outer sheath are made of the same non-crosslinked polymer-based material which is black coloured by addition of carbon black. Using the same base material would allow recycling without the need to separate different materials. Polyethylene is suggested as the polymer material for uses at temperatures below 70xc2x0 C., while thermoplastic elastomers consisting of two-phase mixtures of polypropylene with an ethylene/propylene co- or terpolymer (EPR or EPDM rubber) are proposed in cases involving a maximum working temperature of 90xc2x0 C. Within the latter class of polymers, mention is specifically made of the commercial products Santoprene(copyright) from Monsanto (polypropylene-based thermoplastic elastomer) and of heterophase propylene copolymers obtained in reactor, with an ethylene/propylene elastomeric phase content of greater than 25% by weight, for example 43% by weight of ethylene/propylene rubber (product Novolen(copyright) 2912 HX from BASF). At any event, propylene/EPR or EPDM mixtures are concerned, obtained using conventional vanadium- and/or titanium-based Ziegler-Natta catalysts.
The Applicant has perceived that the technical problem of obtaining a cable with a coating made of a non-crosslinked, and thus recyclable, polymer material which also has mechanical and electrical properties suitable to the usual conditions of use is dependent on the use of a crystalline propylene homopolymer or copolymer mixed with an elastomeric ethylene copolymer having a high capacity for elastic recovery without the need for crosslinking, as indicated by low values of tension set, that is of permanent deformation after a tensile force has been applied to a specimen of non-crosslinked material. Moreover, the Applicant has perceived that this high capacity of elastic recovery in non-crosslinked materials can be obtained with copolymers of ethylene with an alpha-olefin, and optionally with a diene, these copolymers having a highly regular structure, such as those obtainable by copolymerization of the corresponding monomers in the presence of a single-site catalyst, for example a metallocene catalyst.
In particular, the Applicant has found that excellent performances, both in terms of mechanical properties, in particular elongation at break, stress at break and modulus, and in terms of electrical properties, in particular as regards water absorption, can be obtained by using, as non-crosslinked base material for at least one of the coating layers of the cable, a mixture as defined hereinbelow, comprising a crystalline propylene homopolymer or copolymer and an elastomeric copolymer of ethylene with at least one alpha-olefin, and optionally with a diene comonomer, the latter copolymer having a 200% tension set value lower than 30%, preferably lower than 25%.
Therefore, according to a first aspect, the invention relates to a cable comprising a conductor and one or more coating layers, wherein at least one of the coating layers comprises as non-crosslinked base polymer material a mixture comprising: (a) a crystalline propylene homopolymer or copolymer; and (b) an elastomeric copolymer of ethylene with at least one alpha-olefin having from 3 to 12 carbon atoms, and optionally with a diene; the copolymer (b) being characterized by a 200% tension set value (measured at 20xc2x0 C. for 1 minute according to ASTM standard D 412) lower than 30%, preferably lower than 25%.
According to a further aspect, the invention relates to a cable comprising a conductor and one or more coating layers, wherein at least one of the said coating layers has electrical insulating properties and comprises a mixture as defined above as non-crosslinked base polymer material.
According to a further aspect, the invention relates to a cable comprising a conductor and one or more coating layers, wherein at least one of the said coating layers has semiconductive properties and comprises a mixture as defined above as non-crosslinked base polymer material.
According to a further aspect, the invention relates to a cable comprising a conductor and one or more coating layers, wherein at least one of the said coating layers is an outer protective sheath and comprises a mixture as defined above as non-crosslinked base polymer material.
According to a further aspect, the invention relates to a cable comprising a conductor and one or more coating layers, wherein at least 70%, preferably at least 90%, by weight relative to the total weight of the base polymer material of the said coating layers consists of a mixture as defined above.
The polymer mixtures according to the present invention are also capable of containing inorganic fillers without an unacceptable reduction in their mechanical and elastic properties, in particular as to elongation at break, which remains well above 100%. It is thus possible to produce compositions with flame-retardant properties which are endowed with high flexibility and high mechanical strength. The Applicant has also observed that the mixture is more easily processed, as demonstrated by the low torque values measured on filled systems at the end of the mixing process, which are essentially unchanged with respect to mixtures without inorganic fillers.
Thus, according to a further aspect, the present invention relates to a polymer composition with flame-retardant properties, comprising a crystalline propylene homopolymer or copolymer, an elastomeric copolymer of ethylene with at least one alpha-olefin having from 3 to 12 carbon atoms, and optionally with minor amounts of a diene as termonomer, the elastomeric copolymer being characterized by a 200% tension set value of lower than 30%, and preferably lower than 25% (measured at 20xc2x0 C. for 1 minute according to ASTM standard D 412), and an inorganic filler in an amount such as to impart flame-retardant properties.
Moreover, a further aspect of the present invention resides in a cable comprising a conductor, or conductors, and one or more coating layers, wherein at least one of the coating layers comprises a polymer composition with flame-retardant properties as defined above. Additionally, the present invention is directed to a cable having a plurality of conductors and a flame-retardant filler located in the interstices between the conductors.
Further details will be illustrated in the following detailed description, with reference to the appended drawings.